Botrychium pinnatum
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Ophioglossaceae
Botrychium
Botrychium pinnatum H. St. John
Botrychium boreale J. Milde subsp. obtusilobum
(Ruprect) R. T. Clausen
Northwestern moonwort
Tetraploid
Technical description—Plant slender or stout,
3-12 cm high; [common stalk] 3-7 cm tall; the
sterile segment sessile or on large plants short
stalked, not glaucous, once pinnate or on
large plants twice pinnate, the blade oblong or
somewhat narrowed to the tip, the simply
pinnate blades having 7-11 pinnae which are
sessile, broadly winged and confluent with the
rachis, entire or frequently with large rounded
pinnate lobings, the pinnae ovate or elliptical
3-5 mm long, prominently veined with twice
forked veins; sterile segment of large plants
ovate-deltoid, twice pinnate, 4 cm long, 3 cm
wide, the pinnae pinnately cut and closely
resembling the whole blade of a smaller plant;
stalk of the fertile segment exceeding the
sterile segment; fertile segment once to thrice
pinnate, erect, narrow and with ascending
branches; spores rounded tetrahedral 36-45
micrometers in diameter, rugose. (from H. St.
John, 1929) Chromosomes n = 90. (F. S.
Wagner, 1993)
Taxonomy
Botrychium pinnatum was described as a new species in 1929 by Harold St.
John (American Fern Journal 19:11-15) based on specimens from the state of
Washington. Clausen (1938) felt that the taxon was distinct from the European B.
boreale only at the subspecies level and reduced it to B. boreale subsp.
obtusilobum. W. H. and F. S. Wagner (1983) agreed with St. John’s assessment that
this North American taxon was not closely related to B. boreale. Silhouettes of B.
pinnatum are included in comparison of pinnate-pinnatifid moonworts (Wagner and
Wagner, 1983) and report of B. pinnatum in Colorado (Root and Montgomery, 1987).
Identification
Botrychium pinnatum is the only pinnate-pinnatifid to twice pinnate moonwort
of the western contiguous United States that is not somewhat to highly glaucous.
Rather, it has a highly lustrous surface similar to that of B. lanceolatum. Also unlike
most allotetraploids having B. lanceolatum as one parent, the sporophore of B.
pinnatum is regularly pinnate whereas the sporophore of similar species has a
tendency to divide ternately into three large branches as it does in B. lanceolatum.
Plants vary greatly in size with upper pinnae tending to be undivided in small plants,
but all may be recognized by the above characters.
Prior to St. John’s description of B. pinnatum this species was considered a
subspecies of the European B. boreale and many herbarium collections and field
records still bear this label. B. boreale differs from B. pinnatum in having shorter and
broader (more triangular) pinnae with acute tips. This difference is reflected in
Clausen’s (1938) application of the subspecies name obtusilobum. Clausen believed
however that this taxon intergraded with typical B. boreale in British Columbia and
Alaska and thus did not warrant recognition at the species level. Recent studies in
Alaska have clarified this situation with the recognition of a third distinct taxon, B.
alaskense with morphology more or less intermediate between B. pinnatum and B.
boreale (Grant and Wagner, 2002). B. alaskense can be distinguished from B.
pinnatum by its sharply angular pinnae and ternately divided sporophore. Our recent
work with these species indicates that B. boreale does not occur in North America
except in Greenland (Farrar, 2001; Wagner and Wagner, 1993).
Distribution and Abundance
Botrychium pinnatum occurs widely throughout western North America from
high elevations in east-central and northern California, northern Nevada, northern
Arizona, Utah and New Mexico to near sea level in Alaska and northwestern
Canada. Although rare in the southern part of its range, it increases in occurrence
and abundance from Oregon and Montana northward. Though it occurs with many
other moonwort species, it is seldom the most common moonwort at a given site.
In California, B. pinnatum is known from two sites in Siskyou County, one in
Tehama County and one Tuolumne County in Yosemite NP. Considering its relative
abundance in Oregon, this species likely occurs more broadly in northern California.
Habitat
Botrychium pinnatum occurs in a range of habitats including closed canopy
forests, but it is most commonly found in moist grassy sites in open forests and
meadows. It often occurs near streams and other sites where soil moisture is
constant.
Population Genetics
As an allotetraploid, Botrychium pinnatum is fixed for heterozygosity at loci
where its ancestral parents B. lanceolatum subsp. lanceolatum and B. neolunaria
have contributed different alleles. At several of these loci, plants of some populations
display activity of only one of the parental alleles, the other presumably having
experienced a silencing mutation. Through this process the species shows some
genetic variation among populations but it is otherwise remarkably uniform in
genotype throughout its range.
Phylogenetic Relationships
The confusion that has existed between the three taxa, B. pinnatum, B.
boreale and B. alaskense is understandable. Each species has arisen by
allotetraploid speciation from original hybridizations between B. lanceolatum subsp.
lanceolatum and members of the B. lunaria species complex (see discussion of B.
neolunaria ined.). Recent genetic examination of these tetraploids and their putative
parents in Europe and Alaska has disclosed their probable relationships (Stensvold
2008).
The three tetraploid taxa are quite distinct genetically. Alleles contributing to
the genotypes of B. pinnatum are found in American plants of B. lanceolatum and B.
B. neolunaria. Alleles necessary to produce the genotype of B. alaskense are not
present in American B. neolunaria but are present in European plants of B. lunaria.
Furthermore, European genotypes of B. lunaria are present in the Aleutian
archipelago and in high elevations in western Alaska. Stensvold’s conclusion is that
hybridization between American B. lanceolatum and American B. neolunaria has
produced B. pinnatum whereas hybridization between European B. lunaria and
American B. lanceolatum has produced B. alaskense. We hypothesize that
hybridization between European B. lanceolatum and European B. lunaria has
produced B. boreale.
Additional images of Botrychium pinnatum: